The present invention relates to a high frequency heating apparatus using an inverter power-supply in a high tension generating circuit for magnetron driving use, and more particularly to a high-frequency heating apparatus provided with interfaces of both an inverter controlling portion for directly controlling the inverter and a system controlling portion for controlling a heater, an indicator and the like.
Generally, as a result of the recent significant developments in the semiconductor arts and controlling arts, inverter power-supplies are designed having outputs of higher voltages and electric power, especially in the employment of the higher power requirements of semiconductor switching elements.
The inverter power-supplies are being applied in many fields due to the following characteristics:
(1) the power supplies may be made smaller in size and lighter in weight, PA0 (2) the output may be readily varied, PA0 (3) the corresponding operation may be effected without exchanging parts with respect to either of the commercial 50 Hz, 60 Hz AC power source a DC power source such as a DC battery in the input power-supply frequency. PA0 (4) the corresponding operation may be effected with respect to a wider input voltage range, so that it may be effected to 100 V through 200 V in input voltage, to as far as 220 V as the case may be, thus allowing the common use. PA0 (1) a first soft start function for protecting the semiconductor switching elements from the overload, which is caused by the charging current of the capacitor, and PA0 (2) a second soft start function for protecting the semiconductor switching elements from the overlaod at the magnetron rising time. PA0 (1) a timer and momentary power-off detecting function for protection against overload, PA0 (2) a power supply construction which may prevent the user from the danger of being electrified, PA0 (3) the common use of the door opening and closing detection function, and so on.
As all of the above described effects may be achieved through the use of the inverter power-supply for the magnetron driving power-supply in a high frequency heater using the magnetron, the inverter power-supply may be used more and more in the future.
When the inverter power-supply is used as a magnetron driving power-supply, a high voltage of approximately 4 KV and a high power output of approximately 1 KW are required to be fed to the magnetron. A large task in the designing of such an inverter power-supply as described hereinabove is to protect the semiconductor switching elements from an excessive voltage and excessive current. As in the case of a resonance type inverter using a capacitor, a short-circuit current for charging the capacitor flows to the semiconductor switching element, and expecially during the initial driving operation, it is required to protect the semiconductor switching elements from the short-circuit current. This is called a soft start, and this controlling operation is required to be effected until the above described capacitor is charged. Generally, the timer circuit is used, but the problem in this case is that of the timer accuracy, and a so-called momentary power-off detecting function is required for detecting whether or not the capacitor has electrically been discharged by the momentary power-off.
Another problem unique to driving the magnetron is that the magnetron has a heater, and the high current of the magentron does not flow during a period (for a few seconds) before the heater is sufficiently heated. Accordingly, during this period, the semiconductor switching elements are required to be protected, because they are exposed and can be overloaded. Conventionally, a CR timer is used for protection. Even in this case, both the accuracy of the CR timer and a circuit for resetting the CR timer are required.
Namely, the following functions are required.
The first soft start is required during the resetting, several milliseconds after the power off, called momentary power-off, with the latter being unnecessary when the power off is within approximately several hundred millisecond to one second because of remaianing heat of the heater. It is necessary to realize these functions with better accuracy in a simple construction.
a system controlling portion is required to heat the high frequency heating apparatus for a given time period and to display that it is being heated. Such a system controlling portion as described is conventionally composed of a digital circuit using a microcomputer. As this system controlling portion is provided with keyboards to be operated by the user, display portion provided on the operation panel surface, and so on, the service power-supply into the system controlling portion is insulated from the commercial power-supply using an transformer so as to prevent users from any possible risk of being electrified.
The above described inverter controlling circuit has a service power-supply connected to the commercial power-supply so as to drive the semiconductor switching elements connected with the commercial power-supply line.
The inverter controlling circuit has as a danger the possible electrification of the user should he or she touch it. Thus, if the system controlling portion is directly connected with the inverter controlling circuit, such danger cannot be prevented.
In order to provide a high frequency heating apparatus having the above described inverter controlling circuit and the above described system controlling portion at a lower cost, it is necessary to reduce the number of components to a minimum and to simplify the circuit. For example, it is necessary to commonly use, for instance, a door opening and opening detecting means for detecting whether or not the door of the heating chamber is open in both the inverter controlling circuit and the system controlling portion.
As described hereinabove, the following features have to be developed,